(1) Field of the Invention
The present invention relates to a method for the determination of milk production potential in bovines. In particular the present invention relates to a method wherein a level of acetyl coenzyme A synthetase (ACS) in mammary tissue is determined to provide the estimate and wherein the bovine is selected for milk production based upon the ACS level. The method is particularly valuable for estimating potential for milk production in calves at an early date. The method is also particularly useful in breeding selection where the maternal parents exhibit high ACS activity.
(2) Description of Related Art
Molecular mechanisms regulating utilization of substrates in ruminants are not known. The regulation of acetate for milk synthesis at the molecular level is known to be important. Acetate as acetyl coenzyme A (acetyl-CoA) is a major substrate for milk synthesis and occupies a central position in metabolism of Holstein mammary gland as well as in both prokaryotic and eukaryotic cells. Acetyl-CoA is generated via the acetate activation reaction catalyzed by acetyl coenzyme A synthetase (ACS).
The expression of ACS activity in ruminants is unique. ACS activity is constitutive in heart, controlled by nutrition and physiological state in the mammary gland, but not expressed in liver (Mellenberger, B. W., et al., Biochem. J. 136:741 (1973); Marinez, D. I., et al., J. Agric. Food Chem. 24:927 (1976); Ricks, C. A. and R. M. Cook, J. Dairy Sci. 64:2324 (1981)). Ruminant liver is not lipogenic (Emery, R. S., Mobilization, turnover, and disposition of adipose tissue lipids. IN: Digestive physiology and metabolism in ruminants. Ruckebusch, Y. and P. Thivend (eds.) Avi Publ. Co., Westport, Conn., p. 541 (1980)). Therefore, acetate is not utilized by liver. The liver utilizes propionate as the major source of glucose (Reynolds, C. K. et al., J. Dairy Sci. 71:2395 (1988); Amaral, D. M. et al., J. Dairy Sci. 73:1244 (1990)). Since there is a paucity of glucose in ruminants, this pattern of tissue utilization of acetate spares glucose for other vital metabolic functions (Ricks, C. A. and R. M. Cook, J. Dairy Sci. 64:2324 (1981)). ACS is the first committed step in acetate oxidation which is a major source of energy for milk synthesis. Also, in the ruminant mammary gland, ACS is the first committed step in fatty acid synthesis. The activity of ACS is marginal in a dry gland, increases to peak lactation, and then declines in activity as lactation advances (Marinez, D. E., et al., J. Agric. Food Chem. 24:927 (1976)).
In early lactation, blood growth hormone is relatively high and insulin is low. As lactation advances and milk production declines, blood levels of growth hormone decreases and insulin increases. Also, high producing cows have lower blood insulin and higher growth hormone than do low producing cows (Ghirardi, G. G. and R. M. Cook, J. Dairy Sci. 770:(Suppl. 1):49 (Abstr.) (1987)). Insulin inhibits and growth hormone stimulates ACS activity in lactating goat mammary gland. As lactation advances, ACS activity decreases sharply but can be partially reinstated by injecting a combination of growth hormone, prolactin, and dexamethasone (Marinez, D. I., et al., J. Agric. Food Chem. 24:927 (1976)).
ACS activity is directly correlated with milk production. Thus, as lactation advances, less acetate can be utilized. There is a need for identification of cows which may be in need of treatment to augment lactation.
U.S. Pat. No. 5,041,371 to Cowan et al describes an assay and test kit which uses a DNA probe to label a gene encoding a modification of genes adjacent to the gene encoding bovine prolactin which is a protein important to milk production. This method is focused on the presence or absence of the gene and not on the level of expression of the protein. The gene in question is within 1.5 kb of the bovine prolactin gene. Bulls and daughters of bulls are tested for breeding purposes. This method and test kit is marketed commercially.